Liz Russell has not had the easiest time of late. Her husband was unemployed for months, they have two children and a large mortgage. But he's now found a job, and they're trying to put things back together financially. When things were at their tightest, with their savings about to run out, they switched their mortgage to interest-only and slashed spending on everything else. But, crucially, they never defaulted. Credit card repayments were cut to the minimum, but were still paid every month.

Their reward for prudence? The credit card company has shoved their interest rate up from 21.9% to 30.5% – or 61 times the Bank of England base rate.

Liz (not her real name) had her card with Citi. In February 2009 it told her the rate was rising from 15.7% to 21.9%. In April 2010 it wrote again, saying that her card had been sold to another company, called CC Asset Management. And CCAM had decided to raise the interest rate to 26.9% from May.

In October, Citi wrote again, saying goodbye and that things were now fully in the hands of CCAM, who would issue a new card, branded "Opus". But the small print sent by Opus said her rate would be 30.5%. It turns out that the 26.9% in her April letter was a "simple" rate and the 30.5% is a compounded rate, and actually there's no difference, it's just a question of mathematics. Explain that to a customer who took out a card originally at 15.7%.

Liz's balance on her card is £450.32, on her last statement. She made the minimum payment, £13.68, but paid another £9.07 in interest to CCAM. At this rate it will be years before the card is paid off, and CCAM will have enriched itself handsomely. "I feel like I'm being robbed," says Liz.

I decided to take a closer look at CCAM, to see which pockets are lined by Liz's money. It turns out that CCAM is not really a proper company, just a "special-purpose vehicle" set up to buy Citi's 539,000 card accounts and transfer them to another provider, called SAV Credit, which runs Opus. SAV Credit has a website, but doesn't disclose any profit-or-loss figures. At Companies House, its returns reveal that 2008-2009 profits quadrupled. In turn, SAV Credit is itself owned by private equity groups Palamon Capital Partners, Electra and Morgan Stanley Alternative Investments. Electra is publicly quoted, so we know a bit about it, such as that it has handed £1.2bn in cash to its shareholders over the past decade. Palamon is a group with €1.1bn (£924.5m) under management but is rather more secretive.

"Investor reporting" is a closed part of its website but I did discover its chairman spoke at a conference called "SuperReturn International 2008". Maybe the private equity barons can learn more tips about making profits at next year's conference, which features a session entitled: "Who can best exploit the tough market conditions?".

After I contacted Opus, it promised to cut Liz's rate back to 21.9%. Meanwhile others like her have little choice but to carry on paying the sky-high interest rates demanded by private equity. What a super return on their "investment".

• Thanks to David Norman of TCF Investment for exposing a fundamental dishonesty in the reporting of returns that small investors make on unit trusts and the like. Writing in December's Money Management, he tracks down the industry's "vanishing funds". Over the past decade, there have been 2,507 fund launches in the UK – and 2,400 closures and "mergers". When these funds vanish, so does their invariably poor performance. So the historical tables, in aggregating the performance of the better, surviving, funds, distort the actual experience of small investors. His figures indicate that a fund showing 7.3% per annum gains might only have achieved 3% pa.

"A cynic would say it made sense to launch lots of funds, then merge the poor ones back into the lucky one, to make the manager look good," he says.